Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing production of regulated emissions, such as CO2. For example, alcohol and alcohol-based fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies such as turbo-chargers, super-chargers, etc. Further, various approaches may be used to control such alcohol-fuelled engines, including adjustment of boost or spark timing in dependence upon an alcohol content of the engine fuel, and various other engine operating conditions.
One example approach to control alcohol-fuelled engines is described by Brehob in U.S. Pat. No. 7,287,509. Herein, the injection timing of a directly injected alcohol fuel is adjusted to take advantage of the increased charge cooling effects of the alcohol fuel's higher heat of vaporization and increased octane. Specifically, the injection timing of one or more direct injections is advanced with increased alcohol in the fuel to take advantage of the higher latent enthalpy of vaporization of alcohol and to allow more time for vaporization. Further, in some embodiments involving multiple injections, a larger amount of fuel is injected in an earlier injection (such as in an intake stroke) while a smaller amount of fuel is injected in a later injection (such as in a compression stroke or exhaust stroke). By advancing the injection timing, and/or injecting a larger fraction of fuel in an earlier injection, the intake system is cooled to enable the charge density that can be rammed into the combustion chamber to be increased. Overall, the charge cooling effect of the alcohol fuel is used to improve the peak torque output of the engine.
However, the inventors herein have recognized potential issues with such an approach. In one example, during an engine cold-start, when the temperature conditions of the engine are already not hot enough for an efficient combustion, advancing the injection timing responsive to an increase in fuel alcohol content may further cool the system and significantly reduce the efficiency of fuel evaporation and the formation of a homogeneous air-fuel mixture. Injecting a larger fraction of fuel in an earlier injection may further degrade fuel evaporation efficiency. The larger amount of time required to evaporate the fuel may degrade engine startability. Additionally, the charge cooling effect of the alcohol fuel on the intake system may further lower the air-charge temperature at cold-start conditions thereby further degrading combustion stability and increasing potential for engine misfire. As such, this may lead to reduced fuel economy and degraded cold-start exhaust emissions.
Thus in one example, some of the above issues may be addressed by a method of operating an engine cylinder including a direct fuel injector. In one embodiment, the method comprises, during an engine cold start, direct injecting at least some fuel in an intake stroke of the engine and at least some fuel in a compression stroke of the engine, and decreasing a ratio of intake stroke fuel to compression stroke fuel as an alcohol content of the injected fuel increases.
In one example, the engine may be a flex-fuel engine of a vehicle configured with direct fuel injection. During an engine cold-start, when operating the engine with an alcohol-blended fuel, such as during a first number of combustion events from the start of engine rotation, a split fuel injection may be performed with at least some fuel injected in the intake stroke of the cylinder and at least some fuel injected in the compression stroke of the cylinder. The split ratio of the injections, for example, a ratio between an amount of fuel injected in the first intake stroke and an amount of fuel injected in the second compression stroke, may be adjusted based on the alcohol content of the injected fuel. As such, the split ratio may have a value between 0 and 1. Herein, the ratio of intake stroke fuel to compression stroke fuel may be decreased as the alcohol content of the fuel increases. While the split ratio is decreased, the injections may be adjusted so that a start of injection timing in the intake stroke and an end of injection timing in the compression stroke is maintained, even as the alcohol content of the fuel changes, the split ratio changes, and the total amount of fuel injection changes.
The number of combustion events, at engine cranking, over which the split injection with the decreasing split ratio is performed may also be adjusted based on the alcohol content of the fuel. Similarly, a number of engine cylinders for which split fuel injection with the decreasing split ratio is performed may also be adjusted based on the alcohol content of the fuel. In one example, one or more of the number of combustion events since the beginning of engine rotation and the number of engine cylinders may be increased as the alcohol content of the injected fuel increases.
In one example, when operating with a fuel-blend with a lower percentage of alcohol (such as E10, which has approximately 10% ethanol and 90% gasoline), the split ratio may be higher, for example, closer to 1, so that a larger amount of fuel may be injected in the intake stroke while a smaller amount of fuel is injected in the compression stroke. In another example, when operating with a fuel-blend with a higher percentage of alcohol (such as E70, which has approximately 70% ethanol and 30% gasoline), the split ratio may be lower, for example, closer to 0, so that a smaller amount of fuel may be injected in the intake stroke while a larger amount of fuel is injected in the compression stroke). Further, the number of compression stroke injections may be increased as the compression stroke injection amount exceeds a threshold.
By performing multiple injections and adjusting the split ratio of the multiple injections based on the alcohol content of the injected fuel, engine combustion, particularly during engine cranking, may be improved while reducing particulate matter emissions. Specifically, by increasing the proportion of fuel injected in the compression stroke as the alcohol content of the fuel increases, the higher air-charge temperature and higher valve temperature of the engine cylinders during the compression stroke may be advantageously used to more effectively evaporate the directly injected alcohol fuel and form a homogenous air-fuel mixture. In this way, the engine's ability to start with alcohol fuels may be improved. Additionally, by evaporating most of the injected fuel, less fuel may be lost during engine operation, and the need for larger or pilot fuel injections at engine cold-start may be reduced or eliminated. As such, this may provide fuel economy benefits as well as reduced cold-start exhaust emissions. Finally, by maintaining the start of injection timing of the intake injection and the end of injection timing of the compression injection, it is possible to maintain repeatable engine speed profiles.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.